First-principles study of lattice dynamical properties of the room-temperature P21/n and ground-state P21/c phases of

Unveiling the Electronic Structure of Pseudotetragonal WO3 Thin Films

“…Such distortions are important for understanding the vibrational modes and the electronic properties of this system. By combining XRD, TEM, DFT calculations, and ARPES, we determine the role and consequences of the structural distortions on the WO 3 electronic structure, experimentally revealing band splittings as large as 400 meV between the d xz and d xy orbitals and 100 meV between the d yz and d xz orbitals, reminiscent of the proper balance between the amplitude of the M 3 – and X 5 – antipolar modes in different directions. ,,− Finally, we show a large thermal stability for the grown films, and we demonstrate that SOC plays a sizable role in the interpretation of the electronic behavior of WO 3 . Our work not only motivates the use of strain to realize novel structural phases in binary 5d oxides but also shows us how to use it to tune their orbital degrees of freedom.…”

“…WO 3 can be seen as an ABO 3 cubic perovskite with a missing cation. It has, however, never been observed in the reference cubic structure, which exhibits various unstable phonon modes, including antipolar motion of W against O in various directions [X 5 – and M 3 – (see Figure a)] and oxygen octahedral rotations with different tilt patterns (M 3 + and R 4 + ). , Accordingly, in the bulk form, WO 3 undergoes several phase transitions as a function of temperature. Between 1300 and 1500 K, its structure is tetragonal (space groups P 4/ nmm and P 4/ ncc ). , At 1000 K it becomes orthorhombic ( Pbcn ), ,, at room temperature monoclinic ( P 2 1 / n ), , and at 273 K triclinic ( P 1̅), and finally at 200 K, it enters a second monoclinic phase ( P 2 1 / c ), ,,, with no further transitions down to 5 K. This implies that this monoclinic phase is the ground state of bulk WO 3 …”

“…To clarify this issue, we adopted an atomistic approach and performed DFT calculations. To determine the theoretical ground state of the WO 3 film, we focused on the six phases that are observed experimentally in the sequence of structural phase transitions of bulk WO 3 − and explored their energy gain under tensile strain. Starting from the atomic positions of their fully relaxed bulk structures, we fixed their a and b lattice parameters to the pseudocubic a LAO of 0.379 nm while relaxing the c parameter.…”

“…By using AMPLIMODE software, we performed symmetry-adapted mode analysis to identify the distortions, which play a major role in the stabilization of such a Pbcn strained phase. It can be characterized (see Figure a) by (i) octahedral rotations (R 4 + and M 3 + modes) with tilt pattern a 0 b + c – in Glazer’s notation, (ii) an antipolar motion along y (X 5 – mode), (iii) a small contribution of a bending mode (X 5 + ), and (iv) an antipolar motion along the z - and x -axes (M 3 – mode), where the x component of the M 3 – mode appears through anharmonic coupling . This is in contrast with the P 2 1 / c ground state of bulk WO 3 that arises from the contributions of (i) R 4 + with tilt pattern a – a – c – , (ii) antipolar motion along the z -axis (M 3 – ), and (iii) antipolar motion with the same amplitude along the x - and y -axes (X 5 – ) …”

Mechanistic Insights into WO3 Sensing and Related Perspectives

“…There are other phase transitions, but these are not discussed here for the sake of brevity, since they are observed at high temperatures (the interested reader can refer to the detailed description and references in ref. [ 96 ]). The reason for the complex structural behavior of WO 3 can be understood if it is considered that WO 3 possesses the ReO 3 structure, where metal cations occupy the center of the oxygen octahedra sharing the corners.…”